${ 3.98 \times 4.5 = {?}} $
Explanation: ${3}$ ${9}$ ${8}$ ${4}$ ${5}$ ${\times}$ ${.}$ ${.}$ ${0.08}\times {0.5}= {0.040}$ ${0}$ ${0.9}\times {0.5}+{0.04}= {0.49}$ ${9}$ ${3}\times {0.5}+{0.4}= {1.9}$ ${9}$ ${1}$ ${0.08}\times {4}= {0.32}$ ${2}$ ${0}$ ${0.9}\times {4}+{0.3}= {3.9}$ ${9}$ ${3}\times {4}+{3}= {15}$ ${5}$ ${1}$ ${+}$ ${1}$ ${7}$ ${9}$ ${1}$ ${0}$ $\text{The top number has 2 digits to the right of the decimal.}$ $\text{The bottom number has 1 digit to the right of the decimal.}$ $\text{The product has 2 + 1 = 3 digits to the right of the decimal.}$ ${.}$ ${ 3.98 \times 4.5 = 17.910} $